netdev - Re: [PATCH net-next v2 2/6] sfc: perform XDP processing on received packets

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Date:   Mon, 28 Oct 2019 17:11:31 +0000
From:   Charles McLachlan <cmclachlan@...arflare.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
CC:     <davem@...emloft.net>, <netdev@...r.kernel.org>,
        <linux-net-drivers@...arflare.com>
Subject: Re: [PATCH net-next v2 2/6] sfc: perform XDP processing on received
 packets

On 28/10/2019 16:33, Jesper Dangaard Brouer wrote:
> On Mon, 28 Oct 2019 13:59:21 +0000
> Charles McLachlan <cmclachlan@...arflare.com> wrote:

>> +	/* No support yet for XDP metadata */
>> +	xdp_set_data_meta_invalid(&xdp);
>> +	xdp.data_end = xdp.data + rx_buf->len;
>> +	xdp.rxq = &rx_queue->xdp_rxq_info;
> 
> You can optimize this and only assign xdp_rxq_info once per NAPI.  E.g.
> if you "allocate" struct xdp_buff on the callers stack, and pass it in
> as a pointer.

Sadly, this function is about 5 levels deep from the NAPI loop, so I'm not sure 
that swapping one setting of a stack based pointer with many extra passings of 
a pointer will buy us anything. 

> 
>> +	default:
>> +		bpf_warn_invalid_xdp_action(xdp_act);
>> +		/* Fall through */
>> +	case XDP_ABORTED:
> 
> You are missing a tracepoint to catch ABORTED, e.g:
>   trace_xdp_exception(netdev, xdp_prog, xdp_act);
> 
>> +		efx_free_rx_buffers(rx_queue, rx_buf, 1);
>> +		break;
> 
> You can do a /* Fall through */ to case XDP_DROP.

but not if I put the trace_xdp_exception in as well. I think we're always going 
to need two efx_free_rx_buffers calls.

>> +	case XDP_DROP:
>> +		efx_free_rx_buffers(rx_queue, rx_buf, 1);
>> +		break;
>> +	}
>> +
>> +	return xdp_act == XDP_PASS;
>> +}